NEWS: One More Chevy Volt Trick: Reduced Power

October 21, 2010
The Chevrolet Volt's gas-electric powertrain is seriously complex. The mechanicals and four primary driving modes -- which we covered earlier this week -- are fairly straightforward, but the Volt continues to hide curiosities in the computer code and control strategies that define this game-changer's behavior. At yesterday's media drive in suburban Detroit, we learned the details of a rare scenario in which the Volt operates with reduced power during extended-range driving.
Chevrolet Volt Front Three Quarters Static
Reduced power mode exists because of the disparity between the power outputs of the electric motor that drives the wheels and the generator that provides electricity once the gas engine has kicked on. The traction motor is capable of producing 149 hp, while the generator can only turn out 74 hp. Even when the battery has reached "customer empty," it retains 20 percent of its charge, a small sliver of which is used as a buffer when a driver requires more than 74 hp, as they might for a quick acceleration or to climb a large hill. In typical conditions, the generator will refill the buffer range when the driver backs off or the road levels out. But stay on the throttle long enough or find a large enough hill and you'll hit GM's true floor for the battery charge, around 15 percent, that's there to prevent permanent damage to the pack. In this situation, the maximum power is limited to the amount of power that can be transferred directly from the generator to the traction motor. That's 74 hp. To move a 3700-pound car.
Admittedly, only a small percentage of owners will ever experience reduced power in their Volt. The conditions to stress the battery that severely are exceptionally rare in normal driving situations. GM engineers tell us that one real-world scenario for reduced power mode would be a steep grade several miles long, such as you might find in the Rocky Mountains. With just 74 hp available, we're told the Volt would certainly make it up and over the slope, but might do so at, say, 55 mph rather than 70 mph. "You might not be happy at the speed you'd be going," says chief powertrain engineer Pam Fletcher.
2011 Chevrolet Volt Engine
With a sane driver at the helm, the Volt should never experience reduced power mode in a state as flat as Michigan. But we're hardly sane, so we kicked the air conditioning on, dropped the temperature to low, cranked the fan, tapped the heated seats, activated the rear defroster, and raised the stereo volume. Then we lowered the windows to increase the aerodynamic drag. With the accessories set to full power suck, we set about consuming as many kilowatts as safely possible with our right foot. To reduce the amount of energy captured from regenerative braking when it was necessary to brake, we used firm pedal applications to call on the hydraulic binders.
2011 Chevrolet Volt Prototype Side
After a few miles of futility in traffic, an open stretch of road presented itself and we hammered the accelerator. A few strong stops and accelerations later, a warning on the instrument cluster declared, "Propulsion Power is Reduced," but there's no immediate sensation to convince you of that fact. Calling it a "limp" mode would be utterly inaccurate. At least on the flat, Michigan roads, we had no trouble passing a car travelling at 50 mph or accelerating to and maintaining extralegal speeds. Those actions simply required a touch of additional patience -- no big deal in a car that's not exactly fast to begin with. Bring the Volt to a stop, though, and flatfooted acceleration is substantially slower than with full power. Returning to the full power mode requires just a mile or two of relaxed driving, or less than a minute stopped on the side of the road.
While we found the reduced power scenario to be rather inoffensive, Chevy has included a mountain mode for customers who might knowingly or frequently encounter a battery-draining stretch of pavement. It sets "customer empty" at a higher level, so there's a larger buffer to accommodate high loads for a sustained period. Obviously, it'll reduce the pure electric operating range for that particular battery charge and it does require the customer to activate the setting well before they're at the foot of the grade. "It's a feature that's there when you have to drive over something that has the word 'pass' in the name," says Fletcher, referring to roads like Eisenhower Pass and Loveland Pass in Colorado.
Chevrolet Volt Electric Drive Unit

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